Parsing Nested array value in json response problem i

Hi All,
I am trying to parse childId Value from below response in extjs4.1 .I am newbee for association and hasMany concepts .But I am not able to get the value. can anybody tell whether i have defined model correct ?.Can anybody tell what is problem How to fix it..Is am doing right way ?

Something that commonly trips people up is they expect the framework to always know where to find the data meaning what property in your response to use for the data. To specify what property, you should use the associationKey config on the association.